1. Field of the Invention
The present invention relates to a power saving system in a distributed wireless personal area network and a method thereof, and more particularly to a power saving system in a distributed wireless personal area network and a method thereof that can provide a media access control for power saving in a wireless personal area network based on a mobile ad-hoc network.
2. Description of the Related Art
A WPAN (Wireless Personal Area Network) is defined as a network that operates in a personal area of about 10 m. IEEE (Institute of Electrical and Electronics Engineers) participated in determining the standard for such a wireless personal area network. A UWB (Ultra Wide Band) communication technology can provide a transmission rate of more than several hundred megabits per second (Mbps) in such a personal area network. In a WPAN, media are shared among all devices for mutual communications. If possible, the respective devices attempt to be in a power save state to reduce their battery power consumption.
This requires a media access control method for controlling the media access of the devices, which includes, in a broad sense, how to access the network, how to transmit data to other devices at a desired transmission rate, how to optimally use the media, how to detect and dissolve collisions of beacons, and how to optimally use the power.
The media access control method for a WPAN may be classified into a centralized access method and a distributed access method. According to the centralized access method, one device operates for the whole network in order to manage and control the media access for all devices. All devices request the help of a centralized coordinator for their media access such as network participation and channel time allocation. According to the distributed access method, the media access is uniformly distributed over all devices in the network, and all the devices share the burden of managing their mutual media access.
FIG. 1 is a view illustrating a WPAN according to the conventional distributed access method.
Referring to FIG. 1, the WPAN includes many devices that are indicated as points. Circles drawn around the respective devices indicate ranges in which beacons of the corresponding devices are received, respectively. Additionally, the devices included in a circle form a beacon group.
The WPAN based on the distributed access method does not have any centralized coordinator. In the network, a separate dedicated coordinator is not included, but all devices serve as light coordinators that cooperate with one another. Also, the respective devices share information required for performing the media access control such as a channel time allocation, sync method, power saving, etc., for data transmission to other devices. This network system is called an ad-hoc type distributed wireless personal area network system. The respective devices periodically broadcast information about their peripheral devices and information about channel times allocated to the peripheral devices.
The distributed media access control method depends on a timing concept called ‘superframe’. This superframe has a time of a fixed length, and is divided into a plurality of time windows that are called ‘time slots’. These time slots are called MASs (Medium Access Slots).
Some slots are used for the devices to send beacons, and the remaining slots are used to send data. The slots that send the beacons are called ‘beacon slots’, and the slots that send the data are called ‘data slots’. The length of a BP (Beacon Period) may be shorter than the length of a data period. The beacon slots appear along with the start part of the superframe. In addition, the number of beacon slots can be changed according to the number of devices connected.
FIG. 2 is a view illustrating an example of a conventional superframe structure.
The superframe structure as illustrated in FIG. 2 is based on what is defined by the Multiband OFDM (Orthogonal Frequency Division Modulation) Alliance. A superframe is composed of two types of MASs (Medium Access Slots). One type is a beacon slot MAS (a) and the other type is data slot (c). A beacon period (b) is composed of beacon slot MASs according to the number of devices connected in the same beacon group. The remaining part of MASs, which includes the MAS c, constitute a data period (d) composed of media access slots that can be used by devices in the network in order to transfer data to other devices in the network.
256 MASs (i.e., beacon slots and data slots) constitute a superframe of 65.536 ms, and the respective duration of MAS corresponds to 256 μs. Information of the superframe structure can be broadcasted in the beacons being broadcasted by the respective devices. The start time of the superframe is determined by the start of the beacon period, and is defined as a BPST (Beacon Period Start Time).
The devices that belong to the same beacon group use the same beacon period start time for the superframe.
The devices can put information to IEs (Information Elements) such as BPOIEs (Beacon Period Occupancy Information Elements) in beacons, and then the information can be broadcasted to the respective devices that belong to the same beacon group. The information of occupancy state of the beacon slots in the beacon period can be broadcasted through BPOIEs in beacons. The beacon period occupancy information just includes beacon information of the devices that belong to the same beacon group.
Right after the reception of the beacon frame, the device stores a sender's DEVID (Device ID) and a slot number which are in the received beacon. The device also includes this information in the BPOIE to be transmitted during the next superframe. The information of the beacons received during a present superframe is included in the beacon period occupancy information to be sent during the next superframe.
If the device ID of a certain device could not be shown in the beacon period occupancy information of a neighboring device beacon during a predefined number of successive superframes, this means that the corresponding device will change the corresponding beacon slot to an idle slot during the next superframe. Even if the beacon slot is changed, DRP (Distributed Reservation Protocol) can be maintained, and no re-negotiation is required.
In the conventional superframe structure, the MBOA-MAC (Multiband OFDM Alliance Medium Access Control) is defined as two operation modes: an active mode and a hibernating mode which is a power saving mode. In the active mode, a device can be in an awake state or it also can be in a sleep state in order to reduce the power consumption. In the awake state, even if a transmitting part and a receiving part of the device are not in a transmission state and in a reception state, respectively, they consume normal operating power. In the sleep state, the device uses the minimum power by turning off the power supplied to the transmitting part and the receiving part of the device. In the active mode, the devices can switch the awake state to the sleep state and vice versa according to the data reservations pre-declared in the beacon period.
A more efficient power save method is the hibernating mode. The devices in the hibernating mode declare that they will be in the hibernating mode for several superframes through their beacons. In the hibernating mode, the devices are in a deep hibernating state and do not transmit or receive beacons.
Other devices in the corresponding beacon group should give attention to such a declaration, and continuously should include the information about the hibernating devices in their beacon period occupancy information until the hibernating devices awake. Additionally, the devices in the beacon group should maintain the information about the hibernating devices in their local databases, and defer communications with the hibernating devices until the hibernating devices start to operate and send beacons.
However, as discussed in the MBOA MAC v0.5 specification, the conventional method has problems in that if there is any device that does not confirm the beacon through which a certain device has declared its intention to proceed to the hibernating mode, the device does not know when the hibernating device will return to its active mode.
If such a device wants to communicate with the hibernating device, the device should be in an awake state for a long time in order to confirm in which superframe the hibernating device will awake.
By contrast, even if the hibernating device awakes from hibernation mode and enters the active mode, the device does not know whether other devices have proceeded to the hibernating state during the hibernation period of the device itself. Accordingly, the device may continuously remain active for a long time in order to communicate with such devices, and this long waiting time causes the power conservation of the corresponding device to be abruptly decreased.
The above-described situation may occur more frequently in the case of a beacon group having a high-degree of mobility.